This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-274722, filed Sep. 28, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a photomask having a anti-reflection structure.
Pattern transfer is carried out in the following manner in the process that forms a resist pattern by applying a resist on a film to be processed (hereinafter called xe2x80x9cto-be-processed filmxe2x80x9d) on a substrate and performing exposure and development using an exposure apparatus, such as a stepper. The light that has emitted from a light source and condensed by a lens is focused again on the resist by the lens, thereby forming the latent image of the same pattern as the mask (the latent image of a reduced pattern in the case of reduction projection) on the resist. As a result, a mask pattern is transferred on the resist. The mask that is used in this pattern transfer normally has a thin film of a chromium pattern formed on thick glass.
There is a significant difference between refractive indexes of the glass and chromium or refractive indexes of chromium and air. At the time light actually passes the mask, therefore, reflection at the interface between glass and chromium or the interface between chromium and air becomes very large in this structure.
As a solution to this shortcoming, a three-layer structure of chromium oxide/chromium/chromium oxide has been proposed in place of the single layer of chromium as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 23277/1978. The mask with this three-layer structure is designed on the premise of the use of g rays having an exposure wavelength of 435 nm or i rays of 365 nm. With such an exposure wavelength, the use of the three-layer structure drops the reflectance to 10% or less, which shows a sufficient anti-reflection effect.
Recently, however, the exposure wavelength tends to become shorter as patterns become finer and the exposure light is being shifted to KrF excimer laser light with an exposure wavelength of 248 nm or ArF excimer laser light with an exposure wavelength of 193 nm. The aforementioned refractive indexes also vary in accordance with the shift from the longer wavelength to the shorter one. With this difference in refractive index, therefore, the three-layer structure disclosed in the Japanese publication cannot suppress the reflectance to less than 10% and does not demonstrate a sufficient anti-reflection effect.
Specifically, the reflectance at, for example, the interface between glass and chromium oxide or the interface between chromium and air significantly exceeds 10% in the case of KrF excimer laser light of 248 nm or ArF excimer laser light of 193 nm. Therefore, more than 10% of the incident light is reflected at those two interfaces and repeats complex reflection inside the exposure apparatus.
When such light, which is called stray light, reaches the resist through multiple reflection, the stray light, unlike the normal exposure light, has not undergone normal diffraction and thus works as noise. This noise reduces the resolution of the resist, reducing the exposure latitude and focus depth, and further causes adverse influences, such as a change in the pattern shape according to the numerical aperture of the mask.
One way to solve this problem is to eliminate stray light by lowering the reflectance at the aforementioned interfaces and the structure which has a lamination of a anti-reflection film/glass/chromium/chromium oxide/anti-reflection film is actually proposed in Jpn. Pat. Appln. KOKAI Publication No. 51240/1992. This structure does not however reduce reflection at the interface between glass and chromium and demonstrates an inadequate performance as a anti-reflection structure.
The following show the results of an experiment conducted using a conventional photomask.
FIG. 1 is a cross-sectional view exemplifying the structure of the conventional photomask. This conventional photomask is produced by the following method.
First, a chromium oxide film 3 having a thickness of 30 nm, a chromium film 4 having a thickness of 70 nm and a chromium oxide film 5 having a thickness of 30 nm were laminated in the named order on a quartz substrate 1 having a thickness of 0.9 inch by sputtering. Next, a photosensitive resin with a thickness of 0.5 xcexcm was applied on the chromium oxide film 5. Then, this photosensitive resin was exposed with an electron beam and developed, yielding a line and space (L/S) pattern of 0.7 xcexcm/0.7 xcexcm comprised of the photosensitive resin.
Next, with this pattern of the photosensitive resin used as a mask, the chromium oxide film 3, the chromium film 4 and the chromium oxide film 5 were dry-etched using gas which essentially consists of chlorine. Then, the photosensitive resin was removed, thus completing a photomask as shown in FIG. 1.
This photomask was observed with a laser microscope using a Hexe2x80x94Cd laser with a wavelength of 325 nm. It was confirmed that the L/S pattern of the chromium oxide film 3/chromium film 4/chromium oxide film 5 accurately had a dimension of 0.7 xcexcm/0.7 xcexcm.
Next, the light reflectance of this mask was measured with KrF excimer laser light of 248 nm. The reflectance at the time of irradiating light from the bottom side of the quartz substrate 1 was 15.3% and the reflectance at the time of irradiating light from the major surface side of the quartz substrate 1 or from the chromium film 4 side was 13.2%, both being very high.
Further, a pattern was actually formed with the photomask prepared in the above-described manner and the performance of the photomask was inspected under the following conditions. A silicon oxide film 22 with a thickness of 200 nm as a to-be-processed film was formed on a silicon substrate 21 by CVD. Then, a solution of DUV 30 (a product by Brewer Science Limited) was provided with spin-coating method on this silicon oxide film 22 and baked for 90 seconds at 225xc2x0 C., providing a anti-reflection film 23 with a thickness of 60 nm. A resist UV6, produced by Shipley Corporation, was applied on this anti-reflection film 23 and baked for 60 seconds at 130xc2x0 C., yielding a resist 24 having a thickness of 300 nm.
Then, the photomask was exposed at 27 mJ/cm2 using KrF excimer scanner S202A (NA=0.6 and mask magnification of xc3x974), manufactured by Nikon Corporation. Thereafter, the to-be-processed substrate was baked for 90 seconds at 130xc2x0 C. and developed with 2.38% of tetramethylammonium hydroxide (TMAH) for 45 seconds.
FIG. 2 shows an exemplary cross-sectional view of a to-be-processed substrate including the resist pattern that is obtained by the above-described process. While an L/S pattern of 0.175 xcexcm/0.175 xcexcm which was the xc2xc pattern of the photomask pattern was formed as designed, the resist had round corners. Then, exposure was conducted while the focus position is varied. The results showed that the focus depth which made the dimension fall within xc2x110% became very shallow, about 0.4 xcexcm.
As apparent from the results of the experiment, the resolution was deteriorated due to an increased amount of stray light originated from the photomask, and the resist and the exposure apparatus did not demonstrate the intended performances, making the focus depth shallower. In addition, the pattern did not have a rectangular shape.
While the exposure wavelength tends to become shorter as patterns become finer, the above-described conventional method of forming a resist pattern causes the amount of stray light to increase as the reflectance increases in accordance with the shortened exposure wavelength. This results in shortcomings, such as the deterioration of the resolution of the resist.
Accordingly, it is an object of the present invention to provide a photomask which has a low reflectance even when the exposure wavelength becomes shorter.
To achieve the above object, according to a primary aspect of this invention, there is provided a photomask comprising a transparent substrate; a anti-reflection structure having first chromium oxide, chromium and second chromium oxide laminated in order on a major surface of the transparent substrate; and a anti-reflection film formed on a surface of the first chromium oxide and at an interface between the first chromium oxide and the transparent substrate or on at least one of surfaces of the second chromium oxide.
When the anti-reflection film is formed on the surface of the first chromium oxide and at the interface between the first chromium oxide and the transparent substrate, it is desirable that
1.0 less than n less than 2.4, k less than 0.5, 0.01 xcexcm less than d less than 0.3 xcexcm
where n is a relative refractive index with respect air representing a complex refractive index at an exposure wavelength of the anti-reflection film, k is an extinction coefficient thereof and d is a thickness of the anti-reflection film.
When the anti-reflection film is formed on a surface of the second chromium oxide, it is desirable that
1.0 less than n less than 1.55, 0.01 less than k less than 0.3, 0.03 xcexcm less than d less than 0.3 xcexcm
where n is a relative refractive index with respect air representing a complex refractive index at an exposure wavelength of the anti-reflection film, k is an extinction coefficient thereof and d is a thickness of the anti-reflection film.
The anti-reflection film may be a thin film of an organic substance essentially consisting of at least one of carbon, oxygen, nitrogen and hydrogen or a spin-on-glass film.
Because the anti-reflection film is formed on at least one of surfaces of the anti-reflection structure that has chromium oxide, chromium and chromium oxide laminated in order in this invention, it is possible to make the reflectance lower than the one achieved by a photomask which has a anti-reflection structure alone. This ensures an excellent focus depth and provides an excellent pattern shape.
When the anti-reflection film is formed between this anti-reflection structure and the transparent substrate, it is desirable that
1.0 less than n less than 2.4, k less than 0.5, 0.01 xcexcm less than d less than 0.3 xcexcm
where n is a relative refractive index with respect air representing a complex refractive index at an exposure wavelength of the anti-reflection film, k is an extinction coefficient thereof and d is a thickness of the anti-reflection film.
When the anti-reflection film is formed on the opposite surface of the anti-reflection structure to the transparent substrate, it is desirable that
1.0 less than n less than 1.55, 0.01 less than k less than 0.3, 0.03 xcexcm less than d less than 0.3 xcexcm
where n is a relative refractive index with respect air representing a complex refractive index at an exposure wavelength of the anti-reflection film, k is an extinction coefficient thereof and d is a thickness of the anti-reflection film.
The reason for selecting those conditions is based on the following results of an experiment shown in FIG. 11. In FIG. 11, the horizontal scale represents the value of the extinction coefficient k of the anti-reflection film and the vertical scale represents the reflectance. The experiment was conducted under the conditions that the anti-reflection film is formed on the opposite surface of the anti-reflection structure to the transparent substrate and the thicknesses of the individual elements of the anti-reflection structure of chromium oxide/chromium/chromium oxide are respectively 30 nm, 70 nm and 30 nm.
The wavelength of light incident to the photomask is 248 nm, the value of the relative refractive index n of the anti-reflection film is 1.4 and the thickness d=0.22 xcexcm. As apparent from FIG. 11, with k being equal to or less than 0.3, the reflectance becomes equal to or less than 4%, demonstration of a sufficient anti-reflection effect. Likewise, when the value of n or d is shifted, the aforementioned desirable conditions are derived.
According to another aspect of this invention, there is provided a photomask comprising a transparent substrate; a anti-reflection structure having chromium and chromium oxide laminated in order on a major surface of the transparent substrate; a pellicle disposed apart from a surface of the anti-reflection structure by a predetermined distance and formed of a transparent member; and a anti-reflection film formed on a surface of the pellicle.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.